Angle modulation noise squelching system



Feb. 26, 1946., c. w. -l-lANsELl.

ANGLE MODULATION NOISE SQUELCHING SYSTEM .indi/70 INVENTOR.

A Trae/VE Patented Feb. 26, 1946 ANGLE MODULATION NOISE SQUELCHINGSYSTEM Clarence W. Hansell, Port Jefferson, N. Y., assign- Vr to RadioCorporation of America, a corporation of Delaware Application August 1l,1943, Serial No. 498,185

(Cl. Z50-20) 9 Claims.

My present invention relates generally to a noise squelching system forangle modulation receivers, and more particularly to an automaticsquelching circuit arrangement for a receiver of angle modulated carrierwaves.

One of the main objects of -my present invention is to prevent noise oflarge volume in the modulation output from a phase or frequencymodulation receiver when there is no modulated carrier current beingtransmitted from the receiver input terminals to the limiter.

The generic expression angle modulated carrier wave used in thedescription and claims is intended to include frequency modulated andphase modulated carrier waves or hybrids thereof. The term hybridsignifies modulation which possesses characteristics common to bothfrequency and phase modulation.

Another important object of this invention is to provide a simple andeffective method for 0btaining control over a receiver of anglemodulated carrier waves so as to eliminate the rush of noise which isusually associated with a decrease of the received modulated waves belowa usable intensity level; the method comprising the introduction intothe limiter input of just enough locally-produced carrier current tosuppress the noise.

Another important object of my invention is to provide in combinationwith a conventional frequency modulation (FM hereinafter for simplicity)receiver, an auxiliary oscillator whose frequency is spaced from thelimiting frequency of the received signal band by a superaudiblefrequency value; the oscillator energy being iniected into the receiverat a point prior to the limiter input terminals and at such a magnitudethat inter-station noise is suppressed, and the effect upon the outputof the receiver of the auX- iliary oscillator energy itself beingsuppressed, just like noise, in response to the amplitude of said signalattaining a predetermined usable intensity level.

A still more specific object of my invention is to provide an automaticsquelch system for an FM receiver of the superheterodyne type, whereinthere is provided an oscillator operating at a frequency to one side ofthe intermediate frequency (I. F.) of the receiver, the oscillator beingcoupled to the input of the limiter and having its energy level aboutequal to or somewhat greater than the peak values of noise dependingupon the percentage of normal current amplitude at which limitingbegins, and the oscillator frequency preferably being removed in excessof 10,000 cycles from the extreme edge of the pass bandof the I. F.network thereby to prevent audible beats between the oscillator currentand the signal carrier current.

Yet another object of my invention is to kprovide a novel method ofoperating an FMreceiver and rejecting all signals or noise havingamplitudes below the intensity level of an auxiliary carrier wave; theauxiliary Wave being injected into the limiter input thereby to providea noisesuppressing improvement threshold despite the absence of usabledesired signal amplitude.

Still other objects' of my invention are `to improve generally theefficiency and reliability of FM receivers, and more especially toprovide a noise-free FM receiver capable of being economically andsimply manufactured and vassembled.

The novel features which I believe to be characteristic of my inventionare set forthwith particularity in the appended claims; Vthe inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description, takenin connection with the drawing, in which I have indicated schematicallya circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 schematically shows an FM receiver embodying the invention,

Fig. 2 depicts qualitatively the amplitude relations between the desiredsignal band, noise and auxiliary oscillator current at the limiterinput. l

Referring now tothe accompanying drawing, the FM receiving system shownin Fig. 1 has its conventional networks schematically represented. Thoseskilled in the art of radio communication are fully aware of the mannerof constructing and operating an FM receiver adapted to operate on thesuperheterodyne principle. While my invention is not restricted to usein a radio receiver nor to any specific range of frequencies, by way ofspecific example let it be assumed that the receiver shown in Fig. 1 isadapted to be operat d in the present FM broadcast band of 4.2 to 50megacycles (ma). In this band each station is permitted an overallmaximum frequency swing of kilocycles (kc). The operating I. F. valuemay be chosen from a range of 4 to 20 mc. While Ian I. F. value of 8.33mc. is generally found to be a satisfactory compromise for all factors,I have utilized a specific I. F. value of mc. by way of simplifiedillustration.

The FM wave energy is collected by an antenna, and the latter may be ofthe dipole type Y if desired. The collected FM wave energy may 150 kc.The converter l acts to reduce the centei` frequency Fe of the amplifiedFM waves to the lower I. F. value of 5 mc. without changing thefrequency deviation or swing. The I. F. energy may be amplified in oneor more amplier stages, as indicated schematically at 2. Above the I. F.amplifier rectangle 2 I have represented ideal pass band characteristicof the I. F. amplifiers. It will be noted that there is passed a signalenergy band having a width of 150 kc. The mean or center frequency Fc isequal to 5 mc.

The usual amplitude mddulation limiter 3 is employed in order greatly toreduce any amplitude modulation effects which may have developed on theFM Waves in their passage to the input terminals of the limiter. l Thelimiter may be of any conventional and well known type. Its input-outputcharacteristic is ideally shown above the rectangle 3. Generallyspeaking, the limiter has a flat output energy level above apredetermined signal input intensity level. Both grid circuit and platecircuit limiting action may be employed in the limiter tube.

The limited FM wave energy with a mean frequency of Fc at the operatingI. F. value is applied to the FM detector circuit 4. Any conventionaland well known FM detector circuit having the idealized characteristicshown above rectangle 4 may be employed. The detector characteristicrelates' frequency of signal input to detector output voltage. As iswell known, it is common practice to have a pass band width in excess ofthe 150 kc, swing at thel detector input circuit. 'I'he discriminatorsection of the usual FM detector stage acts to translate the FM waveenergy into corresponding amplitude modulated wave energy. The latter isrectified in the rectiiier section to provide the modulation signalsoriginally used .to modulate the carrier at the transmitter station. TheFM `ment threshold) the audio frequency output of the subsequent audioamplifier is essentially noise voltage. The latter, when reproduced bythe reproducer, is very disturbing to the listener. This noisy receptionwill usually occur during tuning between stations, and causes thefamiliar interstation rush of noise in an FM receiver. It will, also,occur as a result of deep fading, since in wave energy supplied to thedetector 4 is a frequency-variable wave. That is,

this case the signal to noise ratio becomes very low.

The production of noise during periods when the signal to noise ratio isbelow the improvement threshold may be explained by a consideration ofthe following phenomena: A limiter is not capable of removing amplitudemodulation having a depth of modulation as great as This is true sincein the condition of 100% modulation the amplitude of the carrier wave iszero for certain intervals. Hence, to provide complete limiting actionthe limiter must supply a signal gain which raises the signal level fromzero to a nite value. This is, of course, impossible. Consequently, whenthe condition of 100% modulation occurs at the limiter input, the outputenergy of the limiter is amplitude modulated regardless of the degree oflimiting obtainable by the limiter. The condition of 100% amplitudemodulation in the FM Wave energy applied to the limiter input terminalsoccurs at the improvement threshold. At this threshold point there is asubstantial equality of the peak voltage of the noise and that of theincoming carrier. Reference is made to the paper by M. G. Crosbyentitled Frequency modulation noise characteristics in the April, 1937,issue of the Proceedings of the Institute of Radio Engineers for a morecomplete analysis of the improvement threshold characteristics.

At the aforesaid point of equality there is a rcomplete cancellation ofthe signal by the noise.

'I'his produces intervals of zero signal. In general, my inventioncontemplates the injection or introduction of an auxiliary carrier intothe limiter input terminals at those instants when there exists acondition of equality, or less than equality, of the peak voltage of thesignal with respect to the peak voltage of the noise. carrier energy isprovided by an auxiliary oscillator 5 whose energy is of a predeterminedfrequency and of constant predetermined amplitude. The auxiliaryoscillations are fed into the receiving system at any desired pointbetween the converter output terminals and the limiter input terminals.Preferably, the auxiliary oscillations are introduced into the receivingsystem at the input terminals of the limiter network. If the I. F.circuits are not too sharply selective the auxiliary oscillator orsquelching current may be introduced at any point ahead of the point oflimiting on the weakest signals. A

As stated previously, the general purpose of the present invention is toprevent noise of large volume in the output from the FM receiverwhenthere is no carrier current being transmitted from the inputterminals of the receiver to the limiter input terminals. My basicmethod is to introduced just enough locally-produced carrier current, ofa frequency other than the desired carrier frequency, to suppress thenoise. The locally-produced carrier current is beyond audible beat witha correctly tuned received carrier current, and preferably beyondaudible beat with current rof any frequency which is not substantiallyreduced in amplitude by frequency selectively in the I. F. amplier. Thelocally-produced carrier current is itself suppressed, just like noise,when a strong enough desired carrier current is received. Morespecifically,l at the input terminals of the limiter there is injectedan auxiliary radio frequency current whose frequency is removed aconsiderable frequency distance from the desired signal frequency, theauxiliary current having an amplitude about equal to, or somewhatgreater than, the peak values of the noise. Accordingly.

The auxiliary when the desired signal carrier disappearsifrom the :inputto the limiter the limiterstill will tbe limiting on the current fromthe auxiliaryradio frequency current source. By this means it iskpossible to secure an automatic noise squelching action.

It should be noted that, to secure complete squelching action, theamplitude of input-energy to the limiter from the auxiliary source minusthe peak values of combined noise currents, should be great enough toproduce a substantial amplitude limiting in the limiter so that littleamplitude modulation of the auxiliary source current will appear in theoutput of the limiter. In practice, of course, the auxiliary currentwill simply be adjusted in amplitude until a desired degree of noisesquelching is obtained. It is necessary in order that beats between theauxiliary current source and the desired signal carrier may produce noaudio output 'from the receiver, that the auxiliary current be removedin frequency by more than, say, at least 10,() cycles from the Aedge ofthe pass band of the I. F. circuit, depending upon the cut-offcharacteristics of the audio system of the receiver, taking into accountthe frequency response characteristics of human ears.

In Fig. l it is shown that to obtain the auxiliary radio frequencycurrent for accomplishing the automatic squelching action there isemployed an oscillator tube operating at a frequency located to one sideof the limiting edge of the pass band of the I. F. network. A practicaladvantage of the present invention resides in the fact that for thistype of noise squelching there is involved an addition of no more tubesthan would be required for other squelch systems used in FM receivers.

In Fig. 2 I have attempted to picture in a purely qualitative and idealmanner the relations which exist between the I. F. signal band, thenoise components and the auxiliary oscillator current. The numeral 6designates the idealized passfband characteristic of the I. F. amplifiernetwork, and it is assumed that the amplitude level of the I. F.signal'band is of a certain predetermined value. The dotted line 'ldenotes the peak noise level and also the level at Which limiting takesplace in the limiter, which is considerably'below the normal amplitudeof the received modulated carrier Wave. The auxiliary oscillator currentis represented by an arrow 8 indicated as being approximately twice thenoise level line l, and is shown as spaced approximately 10 kc. from theright-hand edge of the I. F. pass band 6.

So long as the amplitude of the I. F. signal within band 6 issufficiently above the noise level I there will be no noise reproducedat the receiver reproducer, except that produced due to phase modulationbeats between the noise currents and the signal current, which is smallin magnitude compared With noise output when there is no carrier currentpresent with the noise currents. However, when the amplitude of the I.F. band falls to the improvement threshold point, noise will bereproduced for the reasons given above. The improvement threshold isreached when the amplitude of carrier current in the pass band 6 isapproximately equal to, or a little greater than, the noise level 1, itbeing understood that peak values are being considered and that theaverage of noise currents is substantially less than the peak values.However, when the signal carrier current falls Amuch below theimprovement threshold, or is absent, the auxiliary oscillator current 8will provide an artificial or simulated carrier which will bring thesignal to noise ratio at the limiterinput terminals iabove `theimprovement 'threshold point and thus automatically prevent Athereproduction of noise.

'Should thereceived FM wave diminish in am- '5 plitude substantially tozero there will still be no reproduction of noise, because theauxiliaryoscillator current will simulate the carrier and so far asv thelimiter is concerned there will be offered to its input terminals asignal to noise ratio in l0 excess of the improvement threshold value.By

l5 is so set that only the noise is rejected thereceiver is quietbetween stations during tuning, and produces ,an Aaudi'ofrequency outputWhen a desired signal is present. The very weak signals having anamplitude .close to that of vthe noise are distorted somewhat by thepresence ofthe auxiliary oscillations, and are made somewhat more noisythan when the squelch oscillator is cut out of the circuit. However,this loss isinconsequential since such weak signals would be ratedunserviceable anyway, and should be received, vif

desired, with the squelch circuit shut off.

As stated above, when the received modulated waves increase to or attainan amplitude such as substantially to exceed the noise level and themagnitude of the auxiliary oscillator current,

then automatically the latter current will be `suppressed as if it werenoise. Of course, the oscillator 5 may have its frequency located at apoint spaced from either ofthe limiting `edges of the I. F. pass band.vIts intensity level is preferably predetermined or pre-set for thethermal agitation noise level of the receiver, or for the particularnoise level encountered kin the area where the receiver is beingoperated, if this noise level is higher than the receiver noise level.If desired, the amplitude of squelch current maybe made adjustable bythe listener. 'Ihe numeral 9 indicates an adjusting device which ispreferably included in the auxiliary oscillator network so 4" that theintensity of the auxiliary oscinations may be controlled in order tolcope with the various noise levels encountered in practice. Should itbe found, as might be the case in some types of receivers, that the gainof the tunable high frequency amplifier circuit and converter circuitYis greater at the high frequency end of the tuning range than at thelow frequency end of the tuning range, then it will be desirableconcurrently to adjust the amplitude of the auxiliary oscillations tocompensate for such change of gain.

For example, the tuning device of the receiver could in that case bemechanically coupled to the oscillator amplitude adjusting means 9 sothat the oscillator amplitude level would be set to squelch properly atboth the low and high frequency ends of the tuning device. In this waythe variation in gain of the receiver over the tuning range would becompensated for, since if this were not done the oscillator 5 might verywell squelch properly at the low frequency end of the tuning range andwould not squelch sufficiently at the high frequency end. Any otherwellknown method for compensating for variation in gain over the tuningrange may be utilized. For

instance, a selective equalizer circuit could be inserted in theconverter and prior circuits, or additional limiting could be insertedahead of the point of connection of the squelch oscillator 5.

While I have indicated and described a system for carrying my inventioninto effect, it will be lapparent to one skilled in the art that myinvention is -by no means limited to the particular organization shownand described, but that many -modications may be made without departingfrom the scope of my invention. I l What I claim is: l

l. A method for obtaining control over a receiver of angle modulatedcarrier waves of the type' including a limiter so as to eliminate therush of noise accompanying a decrease of the -received modulated wavesbelow a usable intensity level, which comprises introducing into thelimiter input locally-produced carrier current of a fined frequencysubstantially different from that received from the modulated waves andof sufficient magnitude to suppress the noise.

f2. In a frequency modulation receiver of the type including a limiternetwork; the noise squelching improvement which comprises an os--cillator adapted to produce oscillatory energy having a frequencyspaced from the limiting frequency cf the received signal band by asuperaudible frequency value, and means for injecting the oscillatoryenergy into the receiver at a point prior to the limiter at a magnitudesuch that lauxiliary oscillator operating 1at a, frequency spaced by asuperaudible frequency value from the operating intermediate frequencyof the receiver, means applying the output of said auxiliary oscillatorto the limiter input circuits, and said auxiliary oscillator having itsenergy level about equal to the peak values of noise components in thereceived signal energy.

4. A method of operating a frequency modulation receiver of the typeprovided with a limiter network, which comprises locally producingauxiliary waves having a frequency of the order of the desiredsignalrcarrier but different from the latter, and injecting theauxiliary waves into the limiter network input at a magnitude such as toprovide aneifective signal to noise ratio above the improvementthreshold at the limiterjnput terminals.

5. A phase and frequency modulation receiver having means for preventinga large increase in output due to noise, when there is no substantialsignal current in the receiver circuits, comprising means to introduce acurrei'it having an'amplitude lying between the average amplitude ofnoise currents and the amplitude of least useable signal currents, andsaid introduced current having a, frequency removed from the band ofsignaling currents by an amount equal to or greater than the highestimportant modulation frequency.

6. In a receiver of angle modulated carrier waves of the type providedwith a limiter; an improvement to eliminate the rush of noiseaccompanying a decrease of the received modulated waves below a usableintensity level, which comprises means for introducing into the limiterinput of the 'receiver locally-produced carrier current of substantiallyconstant frequency and of suicient magnitude to suppress the noise.

7. In a frequency modulation receiver of the type including a limiternetwork; the noise squelching method which comprises producingoscillations having a frequency spaced from an extreme frequency of thereceived signal band by a superaudible frequency value, injecting theoscillations into the receiver at a point prior to the limiter at amagnitude such that inter-station noise is suppressed and deriving fromthe limiter energy of the frequencies of said signal band.

V8. In a frequency modulation receiver of the superhetercdyne typeprovided with an intermediate frequency network feeding into a, limiternetwork; the improvement which comprises an auxiliary oscillatoroperating at a Yfrequency spaced by a superaudible frequency value fromthe operating intermediate frequency of the receiver, means applying theoutput of said auxiliary oscillator to the limiter, and said auxiliaryterminals above the improvement threshold and deriving from the limiteroutput terminals energy of the frequency of said received signals.

CLARENCE W. HANSELL.

